Polyhydroxycarboxylic acids (aliphatic polyesters), such as polyglycolic acid and polylactic acid, can be degraded or decomposed by microorganisms or enzymes present in natural environments including soils and sea waters and are accordingly noted as biodegradable polymer materials exerting little load to the environments. Further, as polyhydroxycarboxylic acids are degradable and absorbable in vivo, they are also used as polymeric materials for medical use, such as surgical sutures and artificial skins.
Among the polyhydroxycarboxylic acids, polyglycolic acid is excellent in gas-barrier properties inclusive of oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor barrier property and also excellent in heat resistance and mechanical strength, so that the development for various use thereof as a single material or in a composite form together with another resin material is undertaken in the fields of packaging materials, etc.
A polyhydroxycarboxylic acid can be synthesized by dehydro-polycondensation of a hydroxycarboxylic acid, such as glycolic acid (hydroxyacetic acid) or lactic acid (hydroxypropanoic acid), whereas in order to effectively produce a high-molecular weight aliphatic polyester, there has been generally adopted a process of synthesizing a bimolecular cyclic ester of the hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, the ring-opening polymerization of glycolide (i.e., bimolecular cyclic ester of glycolic acid) provides polyglycolic acid, and the ring-opening polymerization of lactide (i.e., bimolecular cyclic ester of lactic acid) provides polylactic acid.
In any case, as a starting material for a polyhydroxycarboxylic acid with a high molecular weight and little abnormal linkage content, a hydroxycarboxylic acid is required to have a high purity to some extent, but an industrially available hydroxycarboxylic acid is inevitably accompanied with impurities actually. For example, glycolic acid obtained by carbonylation of formaldehyde in water, in the presence of an organic acid and sulfuric acid as catalysts, contains glycolic acid dimer or oligomer formed by ester-forming dehydrocondensation of glycolic acid and di-glycolic acid (OCCH2COOH)2) that is a dimer formed by ether-forming dehydrocondensation of glycolic acid as major impurities in addition to residues of the catalysts. Then, minor components such as the catalyst residues and ionic impurities can be easily separated and removed industrially by such means as adsorption or ion exchange, but a separate means is required for removal of organic impurities. For example, Patent document 1 listed below describes that a 70% industrial-grade glycolic acid aqueous solution typically shows the following composition:
glycolic acid62.4 wt. % glycolic acid dimer8.8 wt. %di-glycolic acid2.2 wt. %methoxyacetic acid2.2 wt. %formic acid0.24 wt. %. 
As general methods for purification or refining by separation of organic materials, unit operations, such as distillation and crystallization, are known. The application of such a purification or refining method to purification of a hydroxycarboxylic acid is, however, accompanied with an inherent difficulty that a hydroxycarboxylic acid readily causes polycondensation under heating. In view of this, distillation involving heating as an essential factor has been considered basically difficult to be adopted, and the purification has been conventionally performed principally by way of crystallization (Patent documents 1-3 listed below). The purification of a hydroxycarboxylic acid by crystallization is generally performed by cooling of an aqueous solution. However, as the solubility of a hydroxycarboxylic acid, particularly glycolic acid and lactic acid, in water is very high, the cooling is required down to 10° C. or below at the minimum or down to below the ice point for recovering the crystal at a high yield, thus requiring a large-scale refrigeration system. Further, the purity of the crystal depends on the operation of solid-liquid separation, so that a large-scale solid-liquid separation apparatus, such as a centrifuge. Thus, the purification by crystallization involves a difficulty that a large-scale apparatus and a high running cost are required, thus resulting in a high purification cost.
With respect to lactic acid having a lower thermal polycondensability than glycolic acid, there has been proposed a method of performing the distillation while replenishing water in order to keep a low concentration of the lactic acid in the aqueous solution thereof (Patent document 4 below). However, this distillation method for purification of hydroxycarboxylic acid is accompanied with an energy loss due to distillation of a large volume of water which has a lower boiling point and a higher vaporizability than hydroxycarboxylic acid and also an increased cost for recovering the hydroxycarboxylic acid by way of condensation of the resultant hydroxycarboxylic acid aqueous solution, thus also involving a problem regarding the purification cost.
Patent document 1: JP-A 6-501268
Patent document 2: WO2003/064366
Patent document 3: JP-A 2006-169185
Patent document 4: JP-A 2002-128727